This invention relates to a cooling system of a liquid-cooled multicylinder engine, particularly a liquid-cooled multicylinder engine of the two cycle type, the cooling system being suitable for use with an engine in which a liquid pump cannot easily be secured to one end of a crank shaft, such as a snow mobile engine having secured to opposite ends of the crank shaft a flywheel of the alternating current ignition system and a belt converter.
The construction and disadvantages of cooling systems of engines of the prior art can be summarized as follows:
1. In one type of cooling system of a liquid-cooled multicylinder engine known in the art, proposals have been made to arrange a liquid pump on one side of the crank case. When the liquid pump is mounted in this position, the pump protrudes sideways from the crank case and increases the bulk of the engine.
2. A cooling system wherein a liquid pump driven by an end of a crank shaft is secured to one end of a crank case and the outlet of the pump is connected directly to the inlet of a liquid passage in the double wall of the crank case which liquid passage has an outlet connected to a liquid jacket in the cylinder at the interface between the crank case and the cylinder and beneath an exhaust conduit has been proposed for use with a liquid-cooled monocylinder engine of the two cycle type. The cooling system of the aforesaid construction offers the advantages that the exhaust conduit and its vicinity are cooled satisfactorily and liquid conduits can be dispensed with. However, since the liquid pump is secured to one end of the crank case, this type of cooling system cannot be applied to a snow mobile engine, for example, which has a flywheel of the alternating current ignition system secured to one end of the crank shaft and a belt converter secured to the other end thereof and in which it is consequently difficult to secure a liquid pump to one end of the crank case. Difficulty also exists in securing a liquid pump to one end of the crank case of an engine of a two-wheeled motorcycle suction system, because parts concerned with suction are mounted at opposite ends of the crank case. Furthermore, in the aforesaid conventional system, a cooling liquid delivered by a liquid pump first flows through a liquid passage in the double wall of the crank case axially of the crank shaft, and then changes its direction of flow upwardly immediately below the suction conduit. In a cooling system having a cooling liquid passage of this construction, the difficulty in uniformly cooling the exhaust conduit and its vicinity increases with each increase in the number of cylinders.
3. In engines of the prior art having more than two cylinders, the wall of the crank case located between the cylinders is constructed to form a housing for bearings journalling the crank shaft. The portion of the wall of the crank case constituting the bearing housing is subjected to heat transmitted from the cylinders, and its temperature rises to a considerably high level (about 150.degree. C.) due partly to the fact that it is difficult to efficiently cool this portion. Thus difficulty is encountered in selecting material, such as rubber for use as an oil seal to be mounted within the bearing housing as an airtight seal member for the crank chambers, and expensive rubber must be used for this purpose.
4. In conventional multicylinder engines of the two cycle type, a crank case formed of a light alloy material is composed of two members, upper and lower. In an engine of this type there is a tendency to produce gaps between the bearing housing and the bearings due to the difference in thermal expansion of the crank case wall, formed of a light metal alloy, and the bearings, caused by a marked rise in temperature in the wall of the crank case which constitutes the bearing housing. When this is the case, a creep is produced in the bearings. In a crank case formed of a light alloy in two parts, it is difficult to provide a large interference between the crank case and the bearings as a measure to prevent this phenomenon. More specifically, if a large interference is provided, the crank case will have an inordinately large thickness and weight, thereby raising the problem of high cost. Additionally, if a large interference is provided, radial gaps between the bearings must be increased, raising the problems of vibration of the crank shaft and noise production by the bearings when the temperature in the crank case becomes high. There have been, for example, cases in which a knock pin is attached to the outer race of the bearing or a weblike ring of nylon is inserted in the outer race of the bearing to cope with the production of a creep. The use of a knock pin has disadantages, however, in that the number of steps in producing the bearings is increased, difficulty is created in assembling the bearing and cost of the bearing increases. There is, moreover, little of benefit to offset all these disadvantages. The insertion of the web-like nylon in the bearing outer race results in reduced strength of the outer race due to the presence of a cutout formed therein for providing a groove for receiving the nylon member, as well as increasing the cost of the bearing.